The present invention relates to a method of manufacturing a magnetoresistive head. In particular, the invention relates to a method of manufacturing a magnetoresistive head using a spin-valve device.
A spin-valve device utilizing the giant magnetoresistive effect (GMR) has been used as a reading device for a magnetic disk drive. A stacked ferri-type spin-valve device referred to as a bottom type comprises an underlayer formed over a lower gap layer, a spin-valve film in which anti-ferromagnetic layer/first ferromagnetic layer/anti-ferromagnetic coupling layer/second ferromagnetic layer/Cu intermediate layer/free layer are stacked on the underlayer, and a protective layer formed over the free layer.
In order to increase the output of the spin-valve device, it is necessary to increase the magnetoresistive effect of the spin-valve film. As the crystal grain size of the spin-valve film increases, the resistance of the spin-valve film decreases to result in increase of the magnetoresistive effect of the spin-valve film.
Further, by increasing the sense current, higher output can be obtained even in a case of using a spin-valve film having an identical magnetoresistive effect. However, when the sense current increases, since the amount of heat generation increases, the characteristics of the spin-valve film are deteriorated by heat. In order to prevent deterioration of the characteristics of the spin-valve film by the heat, it is effective to enlarge the crystal grain size of the spin-valve film.
In view of the requirements described above, growing of the crystal grain size of the spin-valve film has been performed, and the grain size of the spin-valve film formed by a general sputtering method has a diameter of 10 nm or more and about 20 nm in a larger case.
On the other hand, along with increase in the recording density of the magnetic disk apparatus, the track width of the magnetic head has been narrowed year by year. In a reproducing device, the device width of the spin-valve device corresponds to the track width. In the plane recording density of 100 Gbit/in2, the width of the reading track, that is, the width of the spin-valve device reaches 100 nm or less. Since the device height for the spin-valve device is fabricated to about the same size as that for the device width, the area of the spin-valve device is about 10,000 nm2 or less for the plane recording density of 100 Gbit/in2.
As the growing of the crystal grain size of the spin-valve film and miniaturization of the area of the spin-valve device progress, the number of crystal grains of the spin-valve film contained in one spin-valve device decreases. For example, in a case of manufacturing a device of 10,000 nm2 by using a film with the crystal grain size of 10 nm, the number of crystal grains contained in the device is about 130 but the number of crystal grains decreases to about 30 as the crystal grain size increases to 20 nm.
Magnetic characteristics of the magnetic film undergo a significant effect depending on the crystal orientation. Spin-valve films manufactured generally comprise polycrystals and the crystal orientations are not uniform in the in-plane direction. To decrease the effect caused by variations of the crystal orientation on the magnetic characteristics, it is necessary to increase the number of crystal grains contained in the device. For example, in a case where the device contains only one crystal grain, the magnetic characteristics of the device are determined by the orientation of the crystal. That is, the magnetic characteristics of the device are varied unless the crystal orientations are aligned for all the devices. In a case where a number of crystal grains are contained in the device statistically, since the magnetic characteristics of the device no more depend on the crystal orientation or directions of individual crystal grains, variations of the magnetic characteristics between devices can be decreased. Therefore, it is theoretically desired that 300 or more of crystal grains be contained in one device. However, in order to obtain a high-read output, it is necessary to increase the crystal grain size of the spin-valve film. There exists a trade-off relation between a high-read output and small variations of the characteristics between the devices. In this case, in order to improve the yield of the devices, it is necessary to restrict the distribution of the crystal grain size and control the average value thereof precisely.
Patent Document 1 (Japanese Patent Laid-Open No. 1002-33533) discloses a method of using a crystal growth control layer as a method of controlling the crystal grain size of a spin-valve film. Patent Document 1 teaches that the crystal grain size can be controlled to within 3 to 8 nm by inserting a crystal growth control layer between the underlayer and the spin-valve film or in the spin-valve film, the crystal growth control layer comprising a material at least containing one member in the group of consisting of O, N, H, Cu, Au, Ag, and Rh. It also teaches that the crystal growth control layer may be formed dispersingly within the plane of films to be stacked.
Generally, to control the crystal grain size of the film formed by a film forming method such as sputtering, it is effective to provide unevenness with the substrate. Patent Document 2 (Japanese Patent Laid-Open No. 8-130337) discloses methods of forming unevenness in a substrate to which a magnetoresistive effect film is to be formed. A first method is to use an off-substrate of a single crystal substrate cut along a plane inclined by a predetermined angle from a crystal lattice face. A second method is to form unevenness by a photolithographic process. Since the photolithographic process can accurately control the size of the unevenness, it is suitable as a method of controlling the crystal grain size while decreasing the dispersion of the crystal grain size in the film. Further, the spin-valve film is a stack of extremely thin films and is degraded in magnetic characteristic if the height of the unevenness is increased. Accordingly, since the height for the unevenness is in the order of the atom level, that is, from 1 to 5 Å, the etching method and the photoresist for forming the unevenness have many alternatives.